This invention relates both to a procedure for activating an oscillator which oscillates a Coriolis conduit of a Coriolis mass flowmeter for moving fluids, and to a power supply circuit system for an oscillator of a mass flowmeter for moving fluids, operating by the Coriolis principle.
The method by which a mass flowmeter measures moving fluids, employing the Coriolis principle, is based on a process whereby the Coriolis conduit of the mass flowmeter is caused to oscillate with the aid of an oscillator, which imparts a Coriolis force to the fluid passing through the Coriolis conduit, the said force, when detected, providing a measure of the amount of fluid passing through the Coriolis conduit. The oscillator typically incorporates a system consisting of a coil and a magnet in which either the coil or the magnet is rigidly connected to the Coriolis conduit, as a consequence of which an alternating excitation of this system, i.e. an alternating movement of the magnet into the coil and out of the coil, produces an oscillation of the Coriolis conduit.
Maximum efficacy of this type of oscillatory excitation of the Coriolis line is obtained, inter alia, when the oscillation is essentially excited at its intrinsic natural frequency. The oscillatory amplitude of the Coriolis conduit is typically adapted, with the aid of an amplifier circuit, to the Coriolis force to be obtained in the fluid. To that end, the actual oscillatory amplitude of the Coriolis line is captured by means of a sensor attached to the Coriolis line, the amplitude detected is compared with a setpoint amplitude, and as a function of the deviation of the captured amplitude from the setpoint amplitude a sinusoidal excitation signal is amplified by a corresponding amount and fed to the coil of the oscillator.
However, as the excitation signal is amplified, the result is a not so insignificant energy loss due to heat-induced loss in the amplifier circuitry. This has a particularly negative effect in situations where the available electrical energy constitutes a limiting factor, for instance when the system is battery-operated.
It is therefore the objective of this invention to provide a process for the activation of an oscillator serving to oscillate a Coriolis conduit of a Coriolis mass flowmeter for moving fluids, as well as a power supply circuit system for such an oscillator which makes optimal use of the electrical energy available for feeding the oscillator.
The problem first above mentioned is solved by this invention with a process whereby the oscillator is fed a pulse-width-modulated, pulsed sinusoidal signal. In other words, according to the invention, it is not a continuous but a pulsed sinusoidal signal that is fed to the oscillator. The energy transferred with the signal is controlled by pulse-width modulation of the pulsed sinusoidal signal. Since it is necessary in the case of a pulsed sinusoidal signal to feed essentially the same amount of energy to the oscillator as with a continuous sinusoidal signal, the amplitude of the pulse-width-modulated, pulsed sinusoidal signal will always be greater in all non-zero areas than the amplitude of a continuous sinusoidal signal.
In this fashion, the excitation signal in the case of the pulse-width-modulated, pulsed sinusoidal signal is held at a level substantially closer to the maximum signal strength attainable with the amplifier circuit than is true for the continuous sinusoidal signal. By the same token, optimum efficiency of the amplifier circuit is obtained only in the fully closed or fully open state. In the intermediate amplification ranges, however, significant energy losses are sustained due to heat losses within the amplifier circuit system, although these energy losses diminish with progressively increasing transmittance of the amplifier circuit. It follows that in the case of a pulse-width-modulated, pulsed sinusoidal signal there is significantly more efficacious signal amplification than for a continuous sinusoidal signal, given the lower level of heat loss with pulse-width-modulated pulsed sinusoidal signals. In the areas between the pulses of the pulse-width-modulated pulsed sinusoidal signals, meaning the intervals in which the pulse-width-modulated pulsed sinusoidal signal equals zero, amplification efficiency is optimal anyway since no current is flowing and consequently no current can generate heat loss.
In a particularly advantageous implementation of the process according to this invention, the pulse-width-modulated pulsed sinusoidal signal is generated with the aid of a multiplexer by means of which a pulse-width-modulated signal is multiplied by an unpulsed sinusoidal signal of a constant, predetermined frequency.
The pulse-duty factor of the pulse-width-modulated signal, i.e. the width of the individual pulses of the pulse-width-modulated signal relative to their mutual spacing, is preferably set with the aid of a proportional integral differential (PID) controller. Only the pulse width is changed while their mutual spacing is left constant.
A particularly effective regulation of the pulse-width modulation is obtainable when, in setting the pulse-duty factor of the pulse-width-modulated signal in the PID controller, an allowance is made for an error signal which is derived from the difference between the deflection of the Coriolis line as measured by a sensor and a predetermined deflection setpoint value. Such control is further simplified in desirable fashion by keeping the amplitude of the pulse-width-modulated signal constant.
It is also desirable for the frequency of the pulse-width-modulated signal to be significantly higher than the frequency of the unpulsed sinusoidal signal. Indeed, if the frequency of the unpulsed sinusoidal signal essentially corresponds to the resonant frequency of the Coriolis line, there will be virtually no transfer of vibrations to the Coriolis line within the frequency range of the pulse-width-modulated signal, thus preventing any further energy losses due to spurious oscillations of the Coriolis line.
The problem first above mentioned is also solved, according to this invention, by means of a power supply circuit system in which the oscillator of the mass flowmeter can be activated by the power supply circuit with a pulse-width-modulated pulsed sinusoidal signal. In this case the output end of the power supply circuit is preferably provided with an amplifier by way of which the oscillator is powered.
Also, according to this invention, the power supply circuit system includes a desirable feature in the form of a multiplexer by means of which a pulse-width-modulated signal can be multiplied with an unpulsed sinusoidal signal of a constant, predetermined frequency.
Finally, the power supply circuitry is preferably equipped with a gate array incorporating a PID controller with the aid of which the pulse duty factor of the pulse-width-modulated signal can be selected.